Adaptation speed varies:
Rapidly Adapting Receptors : adapt very quickly and are specialized for signaling changes in a stimulus. Pressure, touch, and smell are rapidly adapting receptors. Slowly Adapting Receptors : adapt slowly and continue to trigger nerve impulses as long as the stimulus persists. These receptors monitor stimuli associated with pain, body position, and the chemical composition of the blood.
Somatic Sensations
These sensations arise from stimulation of sensory receptors embedded in the skin or subcutaneous layer; in mucus membranes of the mouth, vagina, and anus; in muscles, tendons, and joints; and in the inner ear. The highest density of these receptors are located on the tip of the tongue, the lips, and the muzzle.
There are 4 modalities of somatic sensation :
1) Tactile Sensations
These sensations include touch, pressure, vibration, itch, tickle
We may perceive them differently, but they may arise from activation of the same receptors. Tactile receptors in the skin include Meissner Corpuscles, Hair Root Plexuses, Merkel Discs, Ruffini Corpuscles, Pacinian Corpuscles, Free Nerve Endings.
a) Touch : generally, result from stimulation of tactile receptors in the skin or subcutaneous layer.
There are 2 types of rapidly adapting touch receptors:
Meissner Corpuscles : located in the dermal papillae of hairless skin. The corpuscle is an egg- shaped mass of dendrites enclosed by a connective tissue capsule. These receptors are abundant in (fingertips, hands, soles) eyelids, tip of the tongue, lips, teats, clitoris and tip of the penis. Hair Root Plexuses : located on hairy skin and consist of free nerve endings wrapped around hair follicles. These receptors detect movements on the skin surface that disturb hairs.
There are 2 types of slowly adapting touch receptors:
Merkel Discs aka Tactile Discs, Type I Cutaneous Mechanoreceptors : these receptors are saucer- shaped flattened free nerve endings that make contact with merkel cells of the stratum basale. These receptors are abundant in (fingertips, hands) lips and external genitalia. Ruffini Corpuscles aka Type II Cutaneous Mechanoreceptors : these receptors are elongated, encapsulated receptors located deep in the dermis and in ligaments and tendons. These receptors are abundant in skin over joints and are most sensitive to stretching that occurs as digits or limbs move.
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